The present invention relates to an air filter medium, an air filter pack and an air filter unit comprising the same, and a method for producing an air filter medium. In particular, the present invention relates to an air filter medium comprising a porous polytetrafluoroethylene (PTFE) film and having a low pressure loss and a high particle-collection efficiency, an air filter pack and an air filter unit comprising such an air filter medium, and a method for producing such an air filter medium.
With the increase of the integration degrees of semiconductors, and the performances of liquid crystals, the degree of cleanness of clean rooms requires the increasingly high level in these years, and thus air filter units having increased particle-collection efficiencies are sought.
Hitherto, high-performance air filters used in such air filter units, in particular, HEPA, ULPA, etc. are prepared by folding filter media which are prepared from glass fibers by wet paper making methods.
However, it is desired to decrease the pressure loss of the air filter units for further reduction of the ventilation power cost and to increase the collection efficiency to realize the cleaner clean rooms, but it is very difficult to achieve the high quality of the air filter units made of the glass fiber (the high collection efficiency when the pressure loss is the same, or the lower pressure loss when the collection efficiency is the same)
Thus, to produce high performance air filter units, air filter units comprising a PTFE porous film, having the better performance than the glass fiber filter media are proposed. It is reported that the pressure loss of a ULPA comprising the PTFE film decreases to two thirds (⅔) of that of the ULPA comprising the glass fiber filter medium (JP-A-5-202217, WO94/16802 and WO98/26860)
The performances of the air filter units comprising the PTFE porous films can be improved by production methods and processing methods, and the PTFE porous films having better properties are proposed. A sole PTFE porous film having high properties (that is, a porous film to which no air-permeable support member is laminated to form a filter medium) is disclosed in JP-A-9-504737, JP-A-10-30031, JP-A-10-287759 and WO98/26860. These patent publications disclose PTFE porous films having a high PF (Performance Factor), which is one index of the performance of the air filter media.
However, it is practically necessary to laminate an air-permeable support member to the PTFE porous film so that the PTFE film is used as an air filter media, since the strength of the PTFE porous film as such should be increased from the viewpoint of handling properties, and the damage of the filter media should be avoided when the filter medium is shaped in a desired form.
As described above, the sole PTFE porous films having the high PF value are known. For example, JP-A-10-30031 describes the PTFE porous film having a PF value of 30. However, among air filter media comprising the laminate of the PTFE porous film and the air-permeable support member, one having a PF value of 19.8 is known from JP-A-10-30031, and one having a PF value of 21.8 is known from WO98/26850, but one having a PF value of larger than 22 has not been known.
In the case of an air filter unit comprising a pleated air filter medium having a PTFE porous film and an air permeable support member laminated to the porous film, WO98/26860 discloses an air filter unit having a PF value of 90.6, although how to produce it is not disclosed. However, any air filter unit having a PF value of larger than 90.6 or how to produce such an air filter unit is not disclosed.
The first object of the present invention is to provide a high performance air filter medium having a low pressure loss and a high collection efficiency, which comprises a PTFE porous film and an air-permeable support member laminated on at least one surface of the PTFE porous film.
The second object of the present invention is to provide an air filter pack having a low pressure loss and a high collection efficiency, which is prepared by pleating an air filter medium comprising a PTFE porous film and an air-permeable support member laminated on the PTFE porous film.
The third object of the present invention is to provide an air filter unit having a low pressure loss and a high collection efficiency and comprising a frame and an air filter pack, which is prepared by pleating an air filter medium comprising a PTFE porous film and an air-permeable support member laminated on the PTFE porous film, stored in the frame.
The fourth object of the present invention is to provide an air filter unit comprising a frame and an air filter pack, which is prepared by pleating an air filter medium comprising a PTFE porous film and an air-permeable support member laminated on the PTFE porous film, stored in the frame, which unit maintains the high performance and is compact (that is, a low folded height (pleat depth) in an air filter pack comprising the pleated air filter medium).
The fifth object of the present invention is to provide an efficient method for producing the above air filter medium.
According to the present invention, the above objects are achieved by
(1) an air filter medium comprising a porous polytetrafluoroethylene film and an air-permeable support member laminated on at least one surface of said porous film and having a PF1 value exceeding 22, which is calculated according to the following formula:
PF1=[xe2x88x92log(Penetration (%)/100)/Pressure loss (mmH2O)]xc3x97100
xe2x80x83in which the penetration (%)=100xe2x88x92Collection efficiency from a pressure loss (unit: mmH2O) measured when an air is flowed through the air filter medium at a flow velocity of 5.3 cm/sec. and a collection efficiency (unit: %) measured using dioctyl phthalate having a particle size of 0.10 to 0.12 xcexcm;
(2) an air filter pack comprising a pleated air filter medium which comprises a porous polytetrafluoroethylene film and an air-permeable support member laminated on at least one surface of said porous film, and having a PF2 value exceeding 90.6, which is calculated according to the following formula:
PF2=[xe2x88x92log(Penetration (%)/100)/Pressure loss (mmH2O)]xc3x97100
xe2x80x83in which the penetration (%)=100xe2x88x92Collection efficiency from a pressure loss (unit: mmH2O) measured when an air is flowed through the air filter pack at a filter medium passing velocity of 1.4 cm/sec. and a collection efficiency (unit: %) measured using dioctyl phthalate having a particle size of 0.10 to 0.12 xcexcm;
(3) an air filter unit comprising a frame and an air filter pack comprising a pleated air filter medium stored in said frame, wherein said filter medium comprises a porous polytetrafluoroethylene film and an air-permeable support member laminated on at least one surface of said porous film, and said unit has a PF3 value exceeding 90.6, which is calculated according to the following formula:
PF3=[xe2x88x92log(Penetration (%)/100)/Pressure loss (mmH2O)]xc3x97100
xe2x80x83in which the penetration (%)=100xe2x88x92Collection efficiency from a pressure loss (unit: mmH2O) measured when an air is flowed through the air filter unit at a filter medium passing velocity of 1.4 cm/sec. and a collection efficiency (unit: %) measured using dioctyl phthalate having a particle size of 0.10 to 0.12 xcexcm;
(4) an air filter unit comprising a frame and an air filter pack comprising a pleated air filter medium stored in said frame, wherein said filter medium comprises a porous polytetrafluoroethylene film and an air-permeable support member laminated on at least one surface of said porous film and has a PF1 value exceeding 22, said air filter medium is pleated at a folded height (pleat depth) of 30 mm or less, and a pressure loss is at least 1 mmH2O when measured by flowing an air through the air filter unit at a filter medium passing velocity of 1.4 cm/sec.; and
(5) a method for producing an air filter medium comprising the steps of:
laminating an air-permeable support member on at least one surface of a porous polytetrafluoroethylene film while heating, and
cooling said laminate of said support member and said porous film just after lamination.